Lithiated allene

Another type of lithiated allenic ether or sulfide is obtained from 2-alkynyl... 

The stability of the various cumulenic anions depends to a large extent upon the nature of the groups linked to the cumulenic system. Whereas solutions of lithiated allenic ethers and sulfides in diethyl ether or THF can be kept for a limited period at about O C, the lithiated hydrocarbons LiCH=C=CH-R are transformed into the isomeric lithium acetylides at temperatures above about -20 C, probably via HC C-C(Li )R R Lithiated 1,2,4-trienes, LiCH=C=C-C=C-, are... 

C. The mixture was cooled to -70°C and the allene (0.22 mol) was added in 5-10 min while maintaining the temperature between -60 and -70°C. After stirring for an additional 30 min at -60°C the solution was ready for further conversions. In the raetallation with ethyllithium the salts initially present had disappeared almost completely after this period. During the lithiation with commercial butyl-lithium the reaction mixture was continuously homogeneous. The solution of the lithiated allenes should be kept below -60°C and used within a few hours. 

A solution of 0.20 mol of butyl lithium in about 140 ml of hexane was cooled to -6Q°C and 140 ml of dry THF were added. The mixture was cooled to about -80 C (liquid nitrogen bath) and 0.23 mol of the allenic hydrocarbon (see Chapter VI, Exp. 1, 2, 44) was added in 5 min (methylal1ene was added as a 1 1 solution in THF). The solutions were kept for 1 h at -55°C. Into another 1-1 flask (see also Fig. 1, but without a dropping funnel), cooled at -90°C by immersion in liquid nitrogen, was poured a solution of dry carbon dioxide (from a cylinder) in 130 ml of dry THF. This solution was obtained by introducing about 40 g of carbon dioxide (note 1) into the THF at -90°C. The gas inlet was removed from the second flask and the solution of the lithiated allene (still cooled below -60 C) was poured... 

Reaction of lithiated allene with methoxymethyl isothiocyanate afforded 107, after trapping with methyl iodide. The newly formed 107 isomerizes under mild conditions to triene 108. This compound is ideally setup to experience an electrocyclization to dihydropyridine 109. Heating in the presence of acid facilitates aromatization of 109 to pyridines 110. 

Pyrrolo[l,2- ][l,2]oxazines are a class of compounds with very few references regarding synthesis and reactivity. An interesting preparation has been described by intramolecular cyclization of IV-hydroxy pyrrolidines carrying a methoxyallene substituent at C-2 (242, Scheme 32). These compounds were obtained by addition of a lithiated allene to chiral cyclic nitrones 241. Cyclization occurred spontaneously after some days at relatively high dilution (0.05 M). Compounds 243 (obtained with excellent diastereoselectivity) can be submitted to further elaboration of the double bond or to hydrogenolysis of the N-O bond to form chiral pyrrolidine derivatives (Section 11.11.6.1) <2003EJ01153>. 

In contrast to lithiated allenes, the corresponding titanium species and carbonyl compounds furnished the regioisomeric y-addition products [68,69]. Thus, reaction of a-aminoaldehydes 63 with the titanated intermediate 75 gave methoxyalkynes 76, which smoothly cydized in the presence of acid and provided lactones 77, again with high anti selectivity (Scheme 8.21) [69]. The regioselectivity depends on the aldehyde used. 

Allenyllithium reagents are commonly prepared through lithiation of propargylic halides or by deprotonation of alkynes or certain allenes (Eq. 9.1). Lithiated allenes often serve as precursors to stable allenylmetal compounds such as stannanes or silanes. They can also be employed for the in situ synthesis of allenylzinc, -titanium and -boronate compounds, which can be further transformed to substitution products not accessible from their allenyllithio precursors. 

A complementary approach for cross-couplings with allenes was applied by using metallated allene species instead of allenyl halides, which have already been discussed in Sect. 14.2.1. Since allenyllithium compounds are readily available by deprotonation of allenes with n-butyllithium, successful cross-coupling reactions between lithiated allenes such as 54 or 57 and aryl or vinylic halides allowed convenient routes to aryl- and vinyl-substituted allenes, e.g. 55, 58 and 60 (Scheme 14.15) [30],... 

The straightforward generation of lithiated allenes [31], in particular lithiated donor-substituted allenes [32], has opened up smooth and efficient routes to further metallated functionalized allenes. By transmetallation, metals such as magnesium,... 

Addition of the lithiated allene to an imine yielded a mixture of four diastereoisomers (ratio 68/17/12/3 68% ds) which was subjected to a silver nitrate induced cyclization. Product 145 derived from major diastereoisomer 144 was isolated and further converted to (-)-preussin (Scheme 36). 

More recently, lithiated allenes with chiral a-alkoxy groups have been generated and added to aldehydes as well as to a tosylimine with substantial diastereoselec-tivity. 

The allene (54) is produced in the reaction of lithiated allene with tetrasilane dichloride, Cl(SiMc2)4Cl <93JA1591>. 

In principle, the reaction of terminally lithiated allenes with electrophilic reagents may give both the allenic and the acetylenic derivative ... 

The reaction of lithiated allenes with phenylisocyanate or thioisocyante affords the 2,4-disubstituted quinoline <97MC92, 96RCB2873>. 

In another approach, lithiated allene 7.4.16 was coupled with aldehyde 7.4.17 to give adduct 7.4.18, which could be ozonized to give side chain methyl ester in 28% overall yield (273),... 

Vinylallenes (88) may be alkylated in good yield to give the 1-alkylallene (89) with minimal formation of the corresponding alkylated acetylenic isomer. Factors that control the regioselectivity of this reaction have been identified as the addition of 1 mole of HMPA to the 1 1 ether-THF solvent mixture, the reaction being carried out at -70 °C, and with strict control of the time interval between formation of the lithiated allene and its reaction with the halide. 

Note i..The large excess was used to prevent any inetallation of RCH=C=CHC00L1 by the lithiated allenes during the addition of the latter derivatives. 

Lithiated allenes, 22 - IB) o-Lithiated allenic ethers, 23 -(7S) Lithiation of propargyl chloride, 24 - 17) Oilithiated 1-alkynes in THF, 24 - 18) Preparation of the zinc-copper couple, 25. 

In the presence of excess LDA, silyl enol ethers of methyl ketones readily underwent elimination to furnish lithiated allenes, which were then trapped by a variety of electrophiles, such as chlorosilanes, chlorostannanes, and ketones (eq 46). Only silyl... 

Recently, an interesting approach to chain-fluorinated pyrimidines was described (CF3COOH as the source of CF3 group). The method rehed on CCCCN-hN ([5-f1]) cyclization of enamides 628 and 629 with NH4OAC to give pyrimidines 630 and 631 in 66 and 31 % yields respectively (Scheme 132) [373], Compounds 628 and 629 were prepared by generation of the corresponding lithiated allene derivatives 626... 

Like their acetylene analogs, (O-lithiated allenes readily cyclize too. As long as the carbon chain does not exceed a length of five or six atoms (see 158 or 159, Scheme 1-114), the new bond connects the metal-bearing tail with the allene center. In... 

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